1. Field of the Invention
This invention relates to a multi-cylinder internal combustion engine, and more particularly to a multi-cylinder internal combustion engine for using a balancer device to prevent secondary vibration caused by reciprocation of a piston.
2. Description of the Related Art
Among the internal combustion engines for vehicles, such as a series four-cylinder engine with a relatively large exhaust capacity, there is known an internal combustion engine mounted with a balancer device for canceling secondary inertia force which worsens vibration/noise problems.
Secondary inertia force is a type of inertia force that is changed by the number of vibrations that is twice the number of engine revolutions. In principle of operation of the piston crank mechanism of an engine, the maximum speed of pistons is at a point near the top dead center (TDC) of crank movement even when the rotation speed of the crank is constant, as shown in FIG. 8, and movement of the reciprocating masses of the pistons or the like connected to a crankshaft is not obtained as complete simple harmonic vibration (sinusoidal vibration). Consequently, out of four cylinders, the inertia forces of No. 1 and No. 4 cylinders and the inertia forces of No. 2 and No. 3 cylinders having different phases deform as shown in the upper graph of FIG. 9B (dotted line and two-dot chain line), and combinations of such inertia forces create a secondary component.
A balancer device of a related art has a balancer shaft that is integrally mounted with a balancer shaft mass capable of generating inertia force of equal magnitude as, but different phase from, the secondary inertia force (see, for example, Japanese Patent Application Publication No. 2005-9637 (JP-A-2005-9637), Japanese Patent Application Publication No. 2001-289055 (JP-A-2001-289055), Japanese Patent Application Publication No. 05-296019 (JP-A-05-296019), Japanese Patent Application Publication No. 2003-201912 (JP-A-2003-201912), Japanese Patent Application Publication No. 2006-105402 (JP-A-2006-105402), and Japanese Patent Application Publication No. 2003-262253 (JP-A-2003-262253)), wherein the balancer device rotates two balancer shafts in mutually opposing directions in the same phase to offset unnecessary vibration components caused by the rotations of the balancer shafts, as shown in FIG. 10 (see, for example, JP-A-2001-289055, JP-A-2003-201912, JP-A-2006-105402, and JP-A-2003-262253).
Because the balancer shafts of the balancer device rotate at a rotation speed twice as high as that of a crankshaft, bearings of a balancer housing that support the balancer shafts need to be lubricated and cooled sufficiently. Therefore, as with journal parts of the crankshaft, the bearings of the balancer shafts are supplied with oil from a main oil gallery of the engine (see JP-A-2005-9637, for example).
There is also known a balancer device in which a baffle plate for regulating excessive variation of the oil level of an oil pan is provided in the vicinity of the balancer device in order to be able to securely suction the oil within the oil pan into an oil pump even when the vehicle accelerates/decelerates or corners, the baffle plate being formed integrally with a balancer housing (see JP-A-2005-9637, for example).
However, since the multi-cylinder internal combustion engine described above has a constitution in which the baffle plate is located above the rotational centers of the balancer shafts (the crankshaft side) or on the side of the balancer shafts, the oil flows down from the balancer device directly into the oil pan. Therefore, the bearings of the balancer shafts rotating at a rotation speed twice as high as that of the crankshaft are cooled entirely by the circulating and supplied oil, and heat from the balancer housing cannot be dissipated sufficiently. For this reason, a sufficient durability of the balancer device cannot be obtained.